Product Information

Technical Info:

Tolerance and Interchangeability

SDI supplies detectors with tolerance values in accordance with Bands 1 to 5, in accordance with International Specifications, or to other specifications requested by the customer. Band 5 detectors have a tolerance of ± 0.01 ohms at 0 °C, equivalent to =/- 0.025 °C.

Temperature Range

SDI detectors are manufactured for use between the -200 °C to +661 °C. To avoid contamination at temperatures above 300 °C, special care should be taken, particularly with use of metal sheaths.

Point of Calibration

Detectors are supplied with standard 10 mm length leads of platinum or platinum group metal. Resistance value and tolerance is realized 5 mm from the ceramic body. The resistance of typical leads are:

0.43 mm diameter platinum 0.7 milliohms/mm

0.28 mm diameter platinum 1.9 milliohms/mm

0.15 mm diameter platinum 6.3 milliohms/mm

For close tolerance applications, care must be taken in positioning the connecting wires. For a detector with 0.28 mm diameter platinum leads, connecting wires at a point 6 mm from the ceramic body instead of 5 mm will result in a 3.9 milliohm error.

Stability

As part of the manufacturing process, SDI detectors are automatically aged to ensure the highest levels of stability. Detectors operated over the range -50 °C to +450 °C have stability levels unattainable from any other type of detector. Typical resistance at 0°C will not change by more than 0.04% after 10 consecutive shocks from -200 °C to +600 °C.

Self-Heating

When tested in a well-stirred ice bath, the rise in temperature will not exceed 0.3 °C, with 10 milliwatts dissipated in the detector. A 1.6 mm diameter x 25 mm long detector has a self-heating characteristic of 0.015 °C/mW in water flowing at 1 meter per second. In air, with only natural convection, this can result in a reading 20 to 40 times greater. A detector passing a current of 1 mA dissipates 0.1 mW of heat at 0 °C. This will give a self-heating effect of 0.6 milliohms in flowing water and 12 to 24 milliohms in naturally circulating air. Thus a reasonable figure for the measuring current in a naturally circulating air application would be 0.3 mA. Smaller detectors require even less current for the same error level. In general, it is recommended the measuring current not exceed 1 mA.

Vibration

Properly supported, detectors will withstand a vibration level of 30 g over the frequency range of 10 Hz to 1 kHz.

Pressure

The ceramic-body construction of SDI detectors protects them from large changes of pressure. However, since elements are not normally hermetically sealed, they should be protected from contamination by liquids or gases through the use of suitable protection sheaths. Hermetically sealed detectors are available by special order.

Insulation Resistance

A 10,000,000 ohm shunt resistance will cause an error of 1 milliohm at 0 °C, equal to 10% of the total tolerance. Error will increase with detector temperature.

Thermal EMF's

Band 5 detectors have an interchangeability of ±0.025 °C at 0 °C. At an assumed current of 0.3 mA, this represents ± 3 microvolts, or a 10% error in tolerance. In addition, any junctions of dissimilar metals within the element, or between the element and measuring instrument, must be within a temperature variation within ± 0.01 °C. AC energization largely eliminates any EMF problems.

Screening and Electrical Interference

Electrical interference from AC supplies or other sources may also affect detector accuracy and is almost entirely dependent on the installation and type of instrumentation in use.

Immersion Errors and Stem Conduction

For a 25 mm long detector contained in a stainless steel sheath, varying the immersion length from 3 to 4 inches in flowing water causes a resistance change of 3 milliohms. The differential for a variation from 4 to 5 inches is reduced to 1.4 milliohms. Therefore a minimum immersion depth of 6 inches is recommended.

Assembly

Recommended methods for connection to the lead wires are welding, and tin or silver soldering. Solder or braze materials containing lead should not be used. Risk of flux contamination restricts the use of brazing techniques. All materials used must be capable of operating over the temperature range required.

Lead Extension

All of SDI's RTD elements can be supplied with extended leads in 2, 3, or 4-wire configurations for easy installation. For manufacturers this translates into lower assembly, inventory and material costs.

The additional benefit offered by these sub-assembly services is the ability to improve overall product reliability by allowing SDI experts to assure proper connection to the fine diameter leads of a platinum resistance element.